There are many devices for sensing and determining the position of a movable member. For example, a linear variable differential transformer ("LVDT") typically has a primary coil that is inductively coupled to two secondary or output coils via an intermediate movable core member. An a.c. current is provided to the driving coil. The currents induced in the driven coils are reflective of the relative position of the core member, and the output signals of the driven coils are analog values of the driving signal. In modern digital control techniques, this analog signal must first be converted to a digital equivalent prior to use.
Other types of devices include variable-phase output signal devices, such as resolvers. These offer the attractive possibility of providing a direct digital output signal by counting the time delay represented by the relative phase shift. This output time count is inherently a digital value.
There has recently been developed an "inverted" LVDT, dubbed a linear variable phase transducer ("LVPT"). From a structure point-of-view, this appears to be, in large part, a conventional LVDT, albeit operated in a reverse manner. More particularly, two separate exciting signals, of identical waveform but shifted 90.degree. from one another, are supplied to two separate coils. The outputs of these coils are coupled to an output coil through the position of the core. The output coil contains the vector-sum of the two 90.degree. out-of-phase inputs. Information on such an LVPT is shown in certain literature of NWL Control Systems, 2220 Palmer Avenue, Kalamazoo, Mich. 49001, a division of Pneumo Corp. Additional details and improvements of this basic device may be shown or described in U.S. Pat. Nos. 4,134,065, 4,297,698 and 4,282,485.
While analog strain gauges have been widely applied for many years, the reliability and performance of semiconductor strain gauges has now increased to the point where they may now be used for many critical sensing applications.
As indicated above, many closed-loop control systems are digital in nature. If any system component produces an analog output signal, this signal must first be converted to its digital equivalent, before it may be integrated in the control scheme. While various types of analog-to-digital converters are available to accomplish this purpose, these represent an additional processing step and a further and unnecessary expense.
Accordingly, it would generally be desirable to provide an accurate transducer that is sensitive to a physical parameter, and that directly produces a digital output.